DEFENSE: Thallium Isotope Investigation of Paleo Ocean Redox and Carbon Dioxide Removal via Enhanced Rock Weathering

The oxygenation of Earth’s surface fundamentally reshaped global biogeochemical cycles, and surface oxygen levels have played a critical role in the origin and diversification of metazoans. Stable thallium (Tl) isotope systematics are mechanistically linked to the burial of manganese (Mn) oxides, making the system an effective redox proxy to track free oxygen (O2) levels in the ocean. The Mesoarchean and mid Proterozoic are two critical periods of time in geologic history for the evolution of microbial and complex life. In the first half of this thesis, I analyze Tl isotopic compositions of mid Proterozoic black shales and Mesoarchean siliciclastic sediments and perform stochastic modeling of marine Tl isotope mass balance to extract paleo redox conditions that have been poorly constrained. The e205Tl composition of the upper Velkerri Formation is indistinguishable from the crustal value, which implies the global burial of Mn oxides was limited at 1.36 Ga and contemporaneous deep ocean was pervasively anoxic. The strong positive e205Tl values of the 2.95 Ga Sinqeni Formation are not likely to represent global seawater records, but rather preserve a primary signal of localized Mn oxides burial in Mesoarchean marine sediments. Given the free O2 is required to stabilize Mn oxides against reductive dissolution during settling, our results provide strong evidence for the early emergence of oxygenic photosynthesis at least 3 billion years ago.

There is increasing consensus that immediate and deep reductions in greenhouse gas (GHG) emissions are necessary in the coming decades to limit global warming to 1.5 C target (the Paris Agreement). Carbon dioxide removal (CDR) from Earth’s atmosphere is likely to play a significant
role in achieving the climate mitigation goals. The second half of this thesis focuses on enhanced rock weathering (ERW), a negative emission CDR strategy that spreads milled calcium and magnesium rich silicates (or alkaline materials) on croplands or in the ocean to artificially speed up the weathering process and associated atmospheric CO2 removal. A hierarchy of models is adopted to evaluate the CDR potential and environmental impacts of ocean based ERW using natural and synthetic mineral feedstocks. Compared to olivine and basalt, application of alkaline metal oxides (CaO and MgO) leads to higher CDR efficiency with reduced environmental impacts, but deployment at scale faces challenges of substrate limitation and process CO2 emissions. I then perform an analysis of the energetic and economic demands of rock grinding, the most energy
demanding and cost intensive step in the ERW life cycle, and conduct state level assessment of carbon footprints, costs, and energy requirements associated with grinding for the U.S. The results of geospatial analysis highlight the regional differences in deploying grinding and indicate that the
operation of grinding in the U.S. is generally cost effective and energy efficient based on the nation’s average electricity mix.